You don't operate a small, touch-sensitive, handheld mobile device (iPhone, iPad, iPod) the same way you do a larger, keyboard-equipped computer (desktop or laptop). So the operating-system software for each of those 2 major types of devices must necessarily be different in some major respects. Thus Apple gives us iOS for the handhelds and Mac OS X for the computers. It seems that the long-range corporate strategy is to get them to operate more and more like each other as time goes by, and some of that's in evidence with the latest release of iOS, Version 9, due out today.
Good news, as always, is that it's free. Somewhat shakier news is that it's only available via download from the Internet, and it's huge. It'll take quite a while to suck it all into your device, so don't start the process for your iPad or iPhone unless your battery is fully charged or it's plugged into a power source. The article above doesn't mention it specifically, but I expect that the size of iOS 9 may also crowd out the amount of memory left over for other apps, which is ironic, given that one of the big talking points about this release is that it'll let you view multiple apps simultaneously on your handheld devices.
Speaking strictly for myself (Richard S. Russell), I'm dreadfully incompetent at using my iPad's virtual keyboard. I much prefer the actual physical keys on my iMac and MacBook Pro. Therefore I use my iPad almost exclusively as a camera or data-viewing device, not for data creation. Yet I marvel at watching my friends who've become accustomed to their iPhones' dictation capabilities generate content almost as fast as they can talk. Since voice-recognition capabilities are supposed to be notably more powerful in iOS 9 (including a smarter Siri who "knows" you better than ever), bridging the gap between the 2 types of physical devices may be further off into the future than we've expected.